Sintering of sol-gel material

ABSTRACT

The invention relates to an economic method for sintering spherical sol-gel material. Spherical hexavalent oxidic uranium material, such as A.D.U. or UO3-hydrate, is converted into nuclear fuel of high density by means of a sinter stage, which consists in heating-up and cooling at a rate of 10* - 65* C/min. The sinter stage may be preceded by a preheating stage. Whereas the starting material may contain finely divided carbon. Previously only spherical sol-gel material of tetravalent uranium and pressed compacts of ceramic uranium dioxide powder where sintered with the above heating rates. Tetravalent uranium material has disadvantages with respect to hexavalent uranium material.

United States Patent [191 Bruijn et al.

[ SINTERING OF SOL-GEL MATERIAL [75] Inventors: Engelbertus Jacobus Bruijn, Den l-lelder; Fokko Wessel Hamburg, Petten; Johannes Bastiaan Willem Kanij, Zvenaar; Robert de Rooy, Egmond, all of Netherlands [73] Assignee: Reactor Centrum Nederland, The

Hague, Netherlands [22] Filed: May 17, 1972 [21] Appl. No.: 254,038

[30] Foreign Application Priority Data May 17, 1971 Netherlands 7106712 [52] US. Cl 264/.5, 252/301.l R, 252/30l.l S, 423/256, 423/261 [51] Int. Cl G2lc 21/00 [58] Field of Search ..252/301.1 S, 301.1 R;

[56] References Cited UNITED STATES PATENTS 3,342,562 9/1967 St. Pierre 423/261 51 Nov. 5, 1974 3,344,081 9/1967 Elyard et a1 252/301.l R I 3,453,216

7/1969 Fitch et al. 252/30l.1 S

Primary ExaminerStephen J. Lechert, Jr. Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT The invention relates to an economic method for sintering spherical sol-gel material. Spherical hexavalent oxidic uranium material, such as A.D.U. or U0 hydrate, is converted into nuclear fuel of high density by means of a sinter stage, which consists in heating-- up and cooling at a rate of 10- 65 C/min. The sinter stage may be preceded by a preheating stage. Whereas the starting material may contain finely divided car bon.

Previously only spherical sol-gel material of tetravalent uranium and pressed compacts of ceramic uranium dioxide powder where sintered with the above heating rates. Tetravalent uranium material has disadvantages with respect to hexavalent uranium material.

12 Claims, N0 Drawings The invention relates to the sintering of fissile material and more particularly of spherical, uraniumcontaining particles which have been prepared according to a so-called sol-gel process. As a rule this sol-gel material is composed of a solid substance termed A.D.U. consisting of UD .xNH .(2x)I-l O. A.D.U. and similar materials are prepared in globular form by effecting the solidification of a metalliferous aqueous phase, this being achieved by the action of one or more of the influential factors heat, dehydration, hydrolysis by theaction of free ammonia or of ammonia formed by the decomposition of an ammonia donor.

For the metalliferous aqueous phase, use is made of metal hydroxide sols, metal salt solutions, anion deficient metal salt solutions or mixtures of these liquids, mixed with carbon if so desired.

In many cases solutions of uranyl nitrate or aniondeficient uranyl nitrate are employed as initial material for the preparation of spherical nuclear fissile particles.

Examples of methods of preparation for sol-gel material are disclosed, inter alia, in the Dutch Patent Applications Nos. 65-05885, 69-00701 and 69-18435 and the British Patent Specification No. 1,191,047 in the name of Reactor Centrum Nederland (Stichting).

In order to convert sol-gel material and other ceramic fissile material into nuclear fissile material, it was formerly supposed that a complicated and lengthy sintering process was necessary for this purpose.

Prior-art sintering methods are indicated below:

Preparation of U PuO and Th0 shaped parts, starting with powders from which these compounds are formed:

Presintering 20 700C 100C/hour dry H sintering 700 1700C 100C/hour dry H Time required 1700C 1 hour wet H Cooling 1700C 20C 200C/hour dry H Total time required for this process, including the calculated time for changing the gaseous atmosphere, approximately 33 hours.

Preparation of UO /ZrO globules, starting with so]- gel material:

Heating in steam 20 300C 50C/hour Heating in C0 300 900C 50C/hour Heating in H 900 1l50C 50C/h0ur Total time required including cooling and changing of gaseous atmosphere: about 36 hours.

The uranium-zirconium starting material was prepared by atomizing a solution of uranyl nitrate which had been mixed with zirconium oxide hydrate sol, ammonia solution and dextrose.

Conspicuous in these above-mentioned sintering processes are:

1. The very long processing times.

2. The changing of the gaseous atmosphere during processing.

It is due to these conditions that the sintering production capacity of this material is small, whilst in addition it is wellnigh impossible technically to sinter in continuous tunnel ovens.

According to the invention it has surprisingly been found that spherical grains of sol-gel material can be sintered by heating and cooling of the material to be J sintered, at a speed of 65C/minute.

In general the invention permitsof application both to a process with continuous ovens and to one with batch ovens.

According to a method of this kind it is possible, for instance, to sinter sol-gel material continuously in about 6 hours, including heating and cooling, in one and the same gaseous atmosphere. It goes without saying thatlthisoffers great advantages, as a result of which the field of application of sol-gel material and more particularly A.D.U. material is greatly extended. A.D.U. material can be converted into'UO ,OOwith a density 98 percent T.D.

It may be stated in the fullest sense that sol-gel initial material is composed of hydroxide-like gel material from actinide metals such as uranium, thorium, plutonium, americium, curium or metals with a possibly higher atomic number, to which other elements may possibly have been added. Other elements are in fact added to improve the nuclear qualities. Other elements suitable for this are zirconium, hafnium, yttrium or the metals of the rare earths.

A suitable sintering atmosphere for continuous sintering is a hydrogen-containing gas mixture, usually consisting of 25 percent of H and percent of N dried over silica gel and activated A1 0 respectively and de-oxygenated with the aid of a catalyst.

This gaseous atmosphere, in the case of a continuous method, is maintained throughout the entire process.

In experiments with batch ovens, in which the changing of the gaseous atmosphere presents no great difficulty, dry oxygen-free gas is used only during cooling, whereas during heating and sintering the gas may be moistened.

Instead of N one may use some other inert gas, as for instance argon. However, for economic reasons nitrogen is to be preferred.

A suitable temperature for sintering A.D.U. globules in a tunnel oven is about 1,600C. It has been found that at this temperature the speed of passage is a maximum.

It is also possible to obtain a well-sintered material at lower temperatures Z 1,200C).

Especially when a tunnel oven is used, breakage occurs if the globule is of a larger size than 600 um (diameter of U0 globule obtained therefrom). This breakage (about 25 percent) can be avoided by diminishing the speed of passage.

However, the speed of passage had to be diminished to such an extend that the production capacity was only 30 percent of its original yield.

Preheating, for instance in air, calcination of the material to be sintered proved in this case to be a solution to the problem.

The A.D.U. material is in this case heatedfrom a 200C/hour.

After calcination the product obtained is sintered at a temperature between l,200 and l,700C, but preferwith solution A.D.U. globules prior to calcination containing the other elements desired. A similarly method is described in British Patent Specification No. 1,025,276, in the name of Reactor Centrum Nederland (Stichting).

ably at l,600C, with a heating-up speed which varies during sintering from to 65C/min, for instance in (FaICmatIOH y effeqted alr, the a continuous tunnel Oven being converted into UQ It 15 also possible to effect The rapid heating/sintering of gel containing thorium calFlnauon a reducing atmosphere or m Vacuo, oxide is known per se. The initial material contains in whfch methods are YP y for the P P this case 3.5 per cent by weight of volatile material con- 10 ration of carbldes and Carbomtrldessisting approximately of equivalent quantities of nitrate and water. After slow heating to about 450C the hot SINTERING substances formed are rapidly heated to a temperature The results of eight sintering tests are set forth in the of at least l,l50C. following Table A. These sintering tests were carried it is to be noted that the final product consists of 50 15 out in an Philips tunnel oven with a length of 2.80 mepercent of irregularly shaped lumps of oxide having a tres.

TABLE A.

lnitial material Calcination Speed of with average gloto 450C passage Sintering Density Breakage bule diameter of: in air mm/min. temp. C. 72 TD.

800 ,umdl I. no 6.7 1650 990 -30 A.D.U.

800 d) a r x. 2. gm no 6.7 1350 98.- 2 5 A.D.U.

800 ;1.m 3. no 3.1 1300 98.5 15-20 A.D.U.

800 #11101) 4. yes 6.7 1625 990 il A.D.U.

400 Lmd) 5. no 5. 1625 990 11 A.D.U.

100 [,Ll'l'ld) approx. 6. no 6.7 1630 96.0 :1

A.D.U.

100 mzb approx. 7. no 3.1 1630 96.5 :1

A.D.U.

100 rno 8. yes 07 1030 97.5 :1

A.D.U.

diameter from 1 to 2 mm, whilst the remainder consists Observations: of smaller particles formed by the cracking of larger 1. The initial material used was A.D.U. globules, fragments. dried at 80C for 2 hours while exposed to air.

It was, however, not obvious that a method whereby 2. By calcination is meant the heating of A.D.U. from percent of the final product had a small diameter 2 to 5 in abOlIt hours While BXPOSEd to due to cracking, would be suitable for the direct sinterin batches. ing of spherical particles of high quality. 3. Sintering is effected in tunnel ovens in an atmo- The continuous sintering process in its entirety insphere of mixed gas (25 percent of H and 75 percluding heating-up and cooling, takes only about 6 cent of N2')- hours. An important condition is that this process shall e nsity 18 easured In H O with the aid of a be carried out during the entire cycle in a dry and oxypycnometer.

gen-free mixture ofa gas containing hydrogen which up to the present had given bad results with this material.

The importance of this invention lies mainly in the fact that it relates to a sintering process which is very attractive both technically and economically.

It is to be noted that it is possible with this process to start with A.D.U. material containing compounds of other elements. It is likewise possible to impregnate 5. The A.D.U. is sintered in A1 0 boats (quality Alsint-Haldenwanger).

Review of test results Test 4 shows that the percentage of breakages can be drastically reduced by calcination.

In spite of the fact that the speed of passage and the sintering temperature was lowered, the percentage of broken globules in test 3 was not drastically reduced.

Test 4 relates to the sintering of globules having an average diameter of 400 microns. According to the results obtained in test 4, globules with this diameter may, if necessary, be sintered without a calcination stage.

The speeds of passage indicated in Table A correspond to rises in temperature of 30C/minute.

We claim:

1. A method for the conversion of spherical sol-gel fissile material into spherical oxidic, carbidic or carbonitridic nuclear fuel material, by means of a sintering step, comprising heating the sol-gel spheres to the sintering temperature and cooling from the sintering temperature at respectively heating and cooling rates ranging between 10 and 65C/minute, the sol-gel material being a composition prepared by solidification through the action of one or more of the factors heat, dehydration, hydrolysis by the action of free ammonia, or of ammonia formed by the decomposition of an ammoniadonor on a globular aqueous metalliferous aqueous phase, selected from the group consisting of metal hydroxide sols, metal salt solutions or mixtures thereof.

2. A method according to claim 1 wherein the sol-gel material consists of spheres of ammonium diuranate.

3. A method as in claim 1 wherein the sintering is carried out in the presence of a reducing gas.

4. A method as in claim 3 wherein the gas is a dry oxygen-free gass and wherein the same gas is present 6 during heating and cooling.

5. A method as in claim 1 wherein sintering iseffected at temperatures ranging between l,200 and 1,700C.

6. A method as in claim 1 wherein the maximum sintering temperature lies between 1,600 and 1,700C.

7. A method as in claim 1 wherein the sintering stage is preceded by calcination in an oxygenous atmosphere.

8. A method as in claim 7 wherein calcination is effected at a temperature of from 300 to 650C.

9. A method as in claim 7 wherein calcination is effected in an oven in which the material is heated at a heating rate not exceeding 200C/hour from room temperature to a temperature that lies between 300 and 650C.

10. A method as in claim 1 wherein the sintering stage is preceded by a preheating stage in a reducing gas or in vacuo.

11. Sintered fragments and globules prepared according to the method of claim 1.

12. A method for the conversion of spherical uranium-containing, hydroxide-like gel particles tospherical oxidic particles comprising heating the gel particles to a sintering temperature of between 1,200 C and 1,700 C and cooling the resulting oxidic particles, the heating and cooling being carried out at rates between 10C/minute and 65C/minute.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, ,5 Dated November 5, 97

Engelbertus Jacobus Bruijn; Fokko Wessel Hamburg; Inventor(s) Johannes Bastiaan Willem Kanij and Robert de Rooy It is certified that error appears .in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column-l, lineS UD .xN H (2-x)H 0 should read UO .xNH3. (2-x)H O Column 1, line 25' 65-05885 should read 66-05885 Column 2, line 13 I UO2-,OO should read U0 Column 5, line 29 gass should read gas Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks FORM PC4050 (0 6m U ITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3, ,5 Dated November 5, 97

Engelbertus Jacobus Bruijn; Fokko Wessel Hamburg; Inventor(s) Johannes Bastiaen Willem Kanij and Robert de Rooy It is certified that error appears .in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column-l, line8 UD .xNH (2-x)H O should read U0 .xNH3. (2-x)H O 3 Column 1, line 25 65-05885 should read 6645885 Column 2, line 13 UO2I,OO should read U0 Column 5, line 29 gass should read gas Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks FORM PO-IOSO (O-69) USCOMMJN- oojunpoq 

1. A METHOD FOR THE CONVERSION OF SPHERICAL SOL-GEL FISSILE MATERIAL INTO SPHERICAL OXIDIC, CARBIDIC OR CARBONITRIDIC NUCLEAR FUEL MATERIAL, BY MEANS OF A SINTERING STEP, COMPRISING HEATING THE SOL-GEL SPHERES TO THE SINTERING TEMPERATURE AND COOLING FROM THE SINTERING TEMPERATURE AT RESPECTIVELY HEATING AND COOLING RATES RANGING BETWEEN 10* AND 65*C/MINUTE, THE SOL-GEL MATERIAL BEING A COMPOSITION PREPARED BY SOLIDIFICATION THROUGH THE ACTION OF ONE OR MORE OF THE FACTORS HEAT, DEHYDRATION, HYDROLYSIS BY THE ACTION OF FREE AMMONIA, OR OF AMMONIA FORMED BY THE DECOMPOSITION OF AN AMMONIADONOR ON A GLOBULAR AQUEOUS METALLIFEROUS AQUEOUS PHASE, SELECTED FROM THE GROUP CONSISTING OF METAL HYDROXIDE SOLS, METAL SALT SOLUTIONS OR MIXTURES THEREOF.
 2. A method according to claim 1 wherein the sol-gel material consists of spheres of ammonium diuranate.
 3. A method as in claim 1 wherein the sintering is carried out in the presence of a reducing gas.
 4. A method as in claim 3 wherein the gas is a dry oxygen-free gass and wherein the same gas is present during heating and cooling.
 5. A method as in claim 1 wherein sintering is effected at temperatures ranging between 1,200* and 1,700*C.
 6. A method as in claim 1 wherein the maximum sintering temperature lies between 1,600* and 1,700*C.
 7. A method as in claim 1 wherein the sintering stage is preceded by calcination in an oxygenous atmosphere.
 8. A method as in claim 7 wherein calcination is effected at a temperature of from 300* to 650*C.
 9. A method as in claim 7 wherein calcination is effected in an oven in which the material is heated at a heating rate not exceeding 200*C/hour from room temperature to a temperature that lies between 300* and 650*C.
 10. A method as in claim 1 wherein the sintering stage is preceded by a preheating stage in a reducing gas or in vacuo.
 11. Sintered fragments and globules prepared according to the method of claim
 1. 12. A method for the conversion of spherical uranium-containing, hydroxide-like gel particles to spherical oxidic particles comprising heating the gel particles to a sintering temperature of between 1,200* C and 1,700* C and cooling the resulting oxidic particles, the heating and cooling being carried out at rates between 10*C/minute and 65*C/minute. 